The invention relates to a semiconductor device and, more particularly, to an exposure apparatus.
A photolithography process in a manufacturing process for a semiconductor device transfers photomask patterns onto a wafer through an exposure apparatus using light diffraction phenomenon. According to such a diffraction phenomenonand because the diffraction angle is a function of the interval between photomask patterns, patterns with variable critical dimensions (CDs) are formed on the wafer even if the patterns on the photomask have the same target CD. In other words, even if the patterns formed on the photomask have the same CD, a CD difference between an isolated pattern and a dense pattern i.e., an “ID bias” may occur between target patterns formed on the wafer due to a density difference between the patterns.
It can be recognized from FIG. 1 that patterns actually formed on a wafer can have different CDs when different exposure apparatuses 101, 102, and 103 are used. For example, dense patterns, semi-dense patterns, and isolated patterns are formed on a single layer in a memory device such as a DRAM. Accordingly, the CD uniformity of patterns formed on the wafer may be degraded due to a pattern density difference.
Accordingly, in order to improve the CD uniformity of wafer patterns, individual photomasks have to be fabricated according to the types of patterns to be formed with the photomasks e.g., a dense pattern, a semi-dense pattern, and an isolated pattern, and an optical proximity correction process has to be independently performed according to the type of photomask. In this case, an exposure apparatus is selected suitably for each photomask. However, because the CD differences between isolated patterns and dense patterns varies among the different exposure apparatuses 101, 102, and 103, patterns having a uniform CD are rarely formed on a wafer.